The present invention relates to an optical disk for recording and reproducing optical information at high density by using laser beams, etc. and a method of recording the optical information on the optical disk.
A technology that highly dense information is reproduced or recorded by using laser beams is known and is mainly put to practical use as an optical disk. A compact disk (CD) on which musical signals are preliminarily recorded is commercially available as one example of application of the optical disk. This compact disk is exclusively used for reproduction of the musical signals. Thus, users of the compact disk can reproduce the musical signals but cannot record signals on the compact disk or erase the signals from the compact disk. Thus, recently, research and development have been vigorously conducted on a rewritable optical disk so as to obtain a compact disk enabling recording and erasure of the signals.
The rewritable compact disk can be roughly classified into a magnetooptical disk and a phase change disk. In the phase change disk, a recording film is reversibly converted between amorphous state and crystalline state by changing irradiation conditions of laser beams so as to record signals and reproduction is performed by optically detecting difference in reflectance between amorphous state and crystalline state of the recording film. Thus, as compared with the magnetooptical disk, the phase change disk has such great advantages that signals can be reproduced by change of reflectance of laser beams in the same manner as the compact disk and single beam overwriting can be easily achieved through modulation of laser power.
As one example of the phase change type rewritable compact disk, an optical disk is proposed in Bulletin of Symposium on Optical Memory (1988), p.41-42. This prior art optical disk has a structure in which a recording film 16 is interposed between dielectric films 15 and 17 as shown in FIG. 18 and the recording film 16 is made of GeSbTe alloy falling in the hatched composition range of FIG. 17.
When CD signals have been actually recorded on the above mentioned prior art optical disk by a conventional single beam overwriting procedure, distortion of the reproduced wave form is impracticably large. This is because shape of the recording mark is not symmetric with respect to its front and rear ends, namely the recording mark is distorted into a teardrop-shape such that the arcuate front and rear ends of the recording mark are thin and thick, respectively. More specifically, in the case where recording is performed by laser beams having a modulated wave form shown in FIG. 19a, temperature of the recording film is low at the front end of the recording mark and becomes higher towards the rear end of the recording mark due to heat accumulation phenomenon as shown in FIG. 19b. As a result, the recording mark has a teardrop-shape as shown in FIG. 19c. The heat accumulation phenomenon becomes more conspicuous as relative speed between laser spots and the optical disk, i.e. linear speed is reduced further. When linear speed is quite low, for example, 1.2 to 1.4 m/sec. as in the case of the compact disk, distortion of shape of the recording mark also becomes large. This distortion of shape of the recording mark leads to distortion of the reproduced wave form, thereby resulting in increase of jitter. Since especially, an EFM (eight to fourteen modulation) signal acting as CD standard is a pulse width modulation (PWM) signal and length and width of the recording mark of the EFM signal indicate information, distortion of the recording mark forms a leading cause of errors.
In order to solve the above described problem, the present inventors previously proposed in U.S. patent application Ser. No. 546,906 filed on July 2, 1990, a method and an apparatus in which a recording pulse for forming one recording mark is initially converted into a pulse train composed of a plurality of short pulses and then, is recorded. Through subsequent extensive study, the present inventors have found that distortion of the recording mark is reduced further by specifying structure of an optical disk and composition of a recording film of the optical disk and by specifying a recording method and thus, reproduced signals having quality equivalent to that of a compact disk exclusively used for reproduction can be obtained also in the rewritable optical disk.